1. Field of the Invention
This invention relates to analog-to-digital converter circuitry and more particularly to successive approximation calibration apparatus, systems, and methods for dynamic range extension in analog-to-digital converter circuitry for camera and imaging systems.
2. Description of Related Art
In recent years, solutions to difficult mixed signal problems related to dynamic range control in camera and imager devices and systems have been attempted. Particularly, it has been desired to develop low cost and low power approaches to improving the dynamic range of digital images. Further, new solutions to image data acquisition and processing have been attempted to result in visible improvements in image quality. Digital camera image quality improvement are sought for video as well as still image camera systems and imaging systems which use charge-coupled device (CCD) imagers, CMOS imagers, and other kinds of imagers.
It is known that the number of bits required for analog-to-digital conversion of CCD data depends upon the noise floor of a CCD, based upon photon shot noise, dark-current noise, and thermal noise from a CCD output amplifier. A system to capture the CCD output requires a quantization noise level lower than the noise floor. The maximum output of the CCD and the noise floor of the CCD can be used to determine the maximum number of bits required for an analog-to-digital converter to have its quantization noise level below the noise level of the CCD. For a particular CCD, the noise voltage level is estimated at about 150 xcexcVrms. The maximum CCD output voltage is about 800 mV. Based upon these conditions, a 12-bit analog-to-digital converter is useful based upon dynamic range requirements. Unfortunately, a 12-bit converter is costly in terms of power and area.
It is further desirable to achieve enhanced image quality with images having improved detail in both dark and light image regions, while avoiding the penalties of high power consumption and large silicon area usage.
U.S. Pat. No. 4,647,975, entitled xe2x80x9cExposure Control System for an Electronic Imaging Camera Having Increased Dynamic Rangexe2x80x9d describes an electronic imaging system with an expanded dynamic exposure range implemented in two exposure intervals.
It is additionally desirable to maintain output linearity and monotonicity during dynamic range extension for analog-to-digital converter circuitry, so that continuity is maintained between segments of the operational characteristic of the converter circuitry irrespective of proximity to trip points.
According to the present invention, a dynamic range expandable imaging system which has a correlated double sampling system, a variable gain amplifier circuit connected to said correlated double sampling system, an analog-to digital converter connected to said variable gain amplifier circuit, and a shifter containing a predetermined number of bits greater than the digital output width of said analog-to-digital converter is calibrated according to a successive approximation technique to ensure output linearity and monotonicity during dynamic range extension for analog-to-digital converter circuitry, so that continuity is maintained between segments of the operational characteristic of the converter circuitry irrespective of proximity to trip points. A shifter is connected to said analog-to-digital converter for receiving the output bit set of the analog-to-digital converter into predetermined locations in the shifter. Input test signals are injected from a predetermined input circuit for sampling by a correlated double sampling system, above and below a first trip point in VGA input values at which VGA gain shifts have been determined, and the difference in analog-to-digital converter output corresponding to said first trip point is determined. Further, input test signals from a predetermined input circuit are provided for sampling by a correlated double sampling system, above and below a next trip point in VGA input values at which VGA gain shifts have been determined and the difference in analog-to-digital converter output corresponding to said next trip point is determined as a calibration value. According to the present invention, a dynamic range enhancement system (DRES) is provided for an imager device which includes a correlated double sampling (CDS) circuit for receiving the video signal from the CCD imaging device, a variable gain amplifier (VGA)subject to automatic gain control, an analog-to-digital converter (ADC) which digitizes the analog signal received from the VGA, an offset mechanism which adjusts the digital output of the ADC to ensure trip-point monotonicity and linearity, and a shifter for adjusting the bit-width of the digital signal to compensate for a change in the amplification provided by the variable gain amplifier. According to the present invention, dynamic range enhancement is achieved in a signal processing system for an imager device subject to offset correction at predetermined trip-points in the ADC characteristic.